Fuel cell diffusion barrier



July 19, 1955 R. J. REID ET AL FUEL CELL DIFFUSION BARRIER Filed April 20, 1950 OOOOOOOOOO nventot United States atent ii Patented July i9, 1955 FUEL CELL DIFFUSION BARRIER Robert J. Reid, Canal Fulton, and Chris E. Best, Franklin Township, Summit County, Ohio, assgnors to The Firestone Tire & Rubber Company, Akron, Ghia, a

This invention relates to novel diffusion barrier membranes in non-metallic fuel cells such as are employed in aircraft and other vehicles.

The type of fuel cell 'to which the present invention relates consists of a plurality of laminae of elastomeric and/or plastic materials plied together to form a tank of the desired shape for containing hydrocarbon and other fuels and liquids. In such cells there is usually provided one lamina or diffusion barrier of a material which'lh'as an extremely high resistance to the diffusion of the cell contents therethrough. The requirements for any material to be used in such a fuel barrier are rather exacting. On the manufacturing side, the material should be soluble in a commercial, non-toxic volatile solvent so that it may be applied as a liquid coating to the cell at some stage during the manufacture thereof, and dried to form the diffusion barrier, as this is the most convenient method of forming a thin, continuous and seamless vapor barrier lamina. On the performance side, the material must be extremely impervious to the hydrocarbon and other fuels which may be contained in the fuel cell, While at the same time being wholly insensitive to humidity. The material must have good adhesion to the other adjacent laminae in the fuel cell, -and must remain dimensionally stable, tough, flexi- 'ble and strong over long periods of time, and over repeated cycles `of extreme heat and extreme cold. In addition, in the case of fuel cells having sealant laminae therein for the purpose of closing gunfire wounds, the barrier must have favorable gunfire shattering characteristics-i. e., any shattering of the barrier must be confined to the immediate vicinity of the wound. These requirements have led, in practice, to the almost exclusive use of nylon resin for this purpose. Even nylon has certain deficiencies, notably the cold properties leave much to be desired, and the resins are permeable to ethanol which is coming into importance as a fuel and as a constituent of fuels. Likewise it is diicult to obtain adhesion of nylon to the adjacent plies of the fuel cells in which it is employed. It is apparent, therefore, that there exists a considerable field for the introduction of novel diffusion barrier materials of improved properties.

Accordingly lit is an object of this invention to provide a novel diffusion barrier for fuel cells.

Another object is to provide such a diffusion barrier having improved properties, particularly dimensional stability, strength and toughness at extreme of high and low temperatures and humidities.

A further object is to provide such a barrier which will have excellent gunfire characteristics.

A still further object .is to provide novel barrier materials which are soluble in cheap commercial solvents of low toxicity, so that they may be applied to the fuel cells by a simple coating operation.

SYNOPSIS OF THE INVENTION The above and other objects are secured, in accordance with this invention, in a fuel cell barrier composed essentially of a copolymer of isobutene with acrylonitrile, containing from 10% to 60% of isobutene, balance acrylonitrile, the percentages being based on the total Weight of isobutene and acrylonitrile in the copolymer. These copolymers are not described in the published prior art, and the mode of preparation thereof will be set forth more fully hereinbelow. These copolymers are soluble in solvents such as methyl ethyl ketone to form solutions which may be applied by brushing or spraying to the fuel cell during construction, and bond readily (either without any priming adhesive at all, or else by the use of a suitable graded adhesive blend containing a mixture of the copolymer and the material to which it is adhered) with any of the elastomeric and plastic materials ordinarily entering into the construction of non-metallic fuel cells. The barriers of this invention have outstanding resistance to the diffusion of any of the hydrocarbon fuels, including aromatic fuels, ordinarily contained in non-metallic fuel cells, and have excellent strength, toughness, dimensional stability and gunfire characteristics at any temperatures or humdities likely to be encountered in service.

he invention is illustrated in the accompanying drawing, which is a perspective view cf a portion of a wall of a fuel cell constructed in accordance with this invention, with the successive laminae stripped back stepwise to show the arrangement thereof.

THE lSOBUTENE-ACRYLONITRIILE COPOLYMERS These copolymers are not yet described in any publications, being the invention of one of the present applicants and a co-worker, see the copending application of Eberly and Reid, Ser. No. 568,067, led December 13, 1944, now Patent 2,537,626. These copolymers are prepared by emulsifying in water, in the presence of a free-radicalliberating catalyst, a mixture of isobutene and acrylonitrile containing from 10% to 90% of isobutene, balance acrylonitrile, based on the total weight of isobutene and acrylonitrile. The resultant copolymers contain from about 10% to about 40% of isobutene, depending on the ratio of monomers charged and the conditions of polymerization. The acrylonitrile is by far the more reactive of the two monomers, and if special measures are not taken, the copolymer formed initially in the polymerization reaction will contain larger proportions of acrylonitrile than the copolymer formed during the later stages of the reaction, resulting in an inhomogeneous product. Such inhomogeneous copolymers may be used in the practice of this invention, providing their overall compositions lie within the range set forth above, but it is preferable to prepare homogeneous copolymers by an increment technique. Such a technique involves maintaining substantially constant the ratio of the weight of unpolymerized acrylonitrile to the weight of unpolymerized isobutene in the reaction mass by adding increments of acrylonitrile during the course of the reaction.

A convenient method of following the reaction analytically for the purpose of determining the amount of increments of acrylonitrile to be added is based upon the assumption that, during the intervals between increments, the copolymer composition does not depart widely from that of the copolymer formed at the very beginning of the reaction. This proportion is, of course, dependent upon the proportions of the monomeric isobutene and acrylonitrile originally charged into the rcaction mass, in accordance with the equation dM1/dM2=the ratio of acrylonitrile to isobutene in the polymer formed at the time under consideration, here the beginning of the reaction.

Mi=concentration of unpolymerized acrylonitrile in the charge at the time under consideration, i. e. the beginnining of the reaction. I

ri=4.0=reactivity ratio for acrylonitrile.

Mz=concentration of unpolymerized sobutene in the original charge at the time under consideration, i. e. the beginning of the reaction.

m==reactivity ratio for sobutene (actually, this ratio is a finite small number, as polymers have ben prepared containing a molar excess of sobutene; however the assumed value of 0 is accurate enough for the range of interest, and simplies calculations).

All of these quantities are theoretically on a molar basis, but in view of the slight difference between the molecular weights of sobutene and acrylonitrile, the concentrations may be taken on a crude weight basis without undue error. From the amount (obtained by solids determination on the polymerization mass) and calculated composition of the polymer formed at any given time, the respective amounts of acrylonitrile and of sobutene that have been polymerized may be calculated. Subtraction from the amount of acrylonitrile and sobutene originally in the reaction mass or added subsequently gives the respective amounts of the still-unpolymerized acrylonitrile and sobutene in the reaction mass, and sufficient acrylonitrile is then added to bring the ratio of unpolymerized acrylonitrile to unpolymerized isobutene back to its original value. These increments, to give the best results, should be added fairly frequently, say after each or 15% of monomers has polymerized.

The polymerization reaction may be carried out under widely variable emulsion polymerization conditions. The weight-ratio of monomers (acrylonitrile plus isobutene) to aqueous emulsion medium may vary from 1:1 to 1:6. Any of the usual emulsifying agents may be employed, such as sodium fatty soaps on the order of sodium or potassium laurate, myristate, oleate, etc., or the sodium or potassium sulfates of lauryl, myristyl oleyl, palmityl and stearyl alcohols. The polymerization may be carried out over a Wide range of temperatures, conveniently from C. to 75 C. Any of the usual free-radical-generating catalysts may be employed, such as sodium, potassium or ammonium persulfates and perborates, benzoyl peroxide, acetyl peroxide, di-t-butyl peroxide and the like.

APPLICATION AND PROPERTIES OF THE BAR- RIERS OF THIS INVENTION The sobutene-acrylonitrile copolymers employed in accordance with this invention are soluble in a number of commercial volatile solvents, examples of these being the ketones containing from 3 to 6 carbon atoms such as acetone, methyl ethyl ketone, methyl isopropyl ketone, cyclohexanone and the like. They are also soluble in acetonitrile, the nitroparafins, alkyl cyanides and the like. Solutions of the copolymers in these solvents may be used to coat the copolymers onto the appropriate lamina of the fuel cell during construction thereof. The sobutene-acrylonitrile copolymers have some compatibility with the elastomeric butadiene-acrylonitrile copolymers which usually constitute the laminae in fuel cells adjacent the diffusion barriers; good bonding is therefore easily obtained in the practice of this invention. The nylon resins heretofore employed as barriers have required the use of special adhesives to bond the nylon membrane to the adjacent laminae.

The sobutene-acrylonitrile copolymers may be plasticized, if desired, by the addition of plastictizers of the aryl sulfonamide, nitroparaflin, and aromatic nitro ester type, and also by means of methyl phthalyl ethyl glycolate and similar esteratype plasticzers. The alkyl phthalyl alkyl glycolate type of plasticzers are preferred, as they have outstanding resistance to extraction by the fuels which will be contained in the cells. Usually, these plasticzers will be employed in an amount varying from l5 to 50%, based on the total weight of copolymer and plasticizer.

The barriers of this invention have extremely small permeability for any of the fuels ordinarily employed in aircraft, including parainic and aromatic fuels and fuels containing aliphatic alcohols such as ethanol. They retain their strength, flexibility and toughness at the extremes of high and low temperatures to which fuel cells will ordinarily be subjected. Particularly in fuel cells designed for military use, the diffusion barriers of this invention have thev desirable property of not shattering over a wide area when penetrated by gunfire, which both improves the performance of the cell after such penetration and before repair, and also simplifies the repairing operation, since it is only necessary to repair the cell wall in the immediate vicinity of the wound.

With the foregoing general discussion in mind, there is given herewith a detailed example of the practice of this invention. All parts given are by weight.

A. Preparation of the sobutene-acrylonitrile copolymer sodium lauryl sulfate manufactured by the Du Pont Co.) 30 parts. Potassium persulfate 30 parts. Disodium phosphate heptahydrate l (NazHPO4.7HzO) 30 parts.

A jacketed autoclove provided with a rotary stirrer and with a lock for the introduction of supplemental ingredients was employed in this preparation. All of the ingredients listed above, except the sobutene, were charged into the autoclave at room temperature, and the system was evacuated to an absolute pressure of mm. A small amount of sobutene was charged and vented in order to purge the system. The sobutene of the recipe was then added, stirring commenced, and the temperature raised to 50 C., which stirring and ternperature were maintained throughout the subsequent reaction.

Samples of the reaction mass were withdrawn at inter vals and the percentage of solid polymerizate therein determined. As often as the analysis indicated that the unpolymerized acrylonitrile in the reaction mass had been depleted so that its ratio to the unpolymerized sobutene had fallen below the original value, an increment of 200 parts of acrylonitrile was charged into the autoclave.

Set forth herewith in Table I are particulars of the run.

TABLE I In t A Tlotail: il

(Bremen CTY 0111 l e f harged Percent Time (hrs. from inception of o Solids in reaction) Accrlgretue (oglarlus reaction (parts) ments) mass (parts) At the end of the reaction period, the reaction mass was cooled to C., and the unreacted isobutene vented and stripped at 60 mm. absolute pressure. The resultant latex was then discharged from the autoclave and coagulated by addition of disodium phosphate solution. The coagulum was diluted with twice its volume of water at 50 C., filtered, washed on the filter, resuspended in water at 50 C., reltered, and again washed on the filter. The resin was then dried overnight at about 60 C. The copolymer contained 74% acrylonitrile as indicated by nitrogen analysis.

A film cast from an acetone solution of the resin retained a substantial degree of flexibility at 32 C. A tlm .0025 inch thick passed a 40% aromatic aviation fuel at the rate of only .003 uid ounce per square foot per 24 hours at the end of 8 days on a diffusion test in which one side of the lm was exposed to air, and the other side to the fuel. This test gives variable results, but the above figure is well below the Air Force maximum of .02 fluid ounce per square foot per 24 hours.

B. Fabrication and testing of a fuel cell FABRICATION A fuel cell in the form of a cube having a dimension of two feet on each edge was constructed from the plies listed below. The cell was built up in the usual way upon a destructible plasterboard form having an external configuration mating with the internal configuration of the cell to be constructed. Successive laminae in the order as listed below were plied upon the formwith suitable laps at the corners and seams. Most of the plies had tacky character so as to remain in place as built. Usually the plies were freshened, by brushing with a solvent and/or adhesive to increase the tack. Certain of the plies had imbedded reinforcing cords, which are described below as running vertically, horizontally, etc. The vertical direction in any side wall of the cell was taken as the vertical direction of the wall as the cell was to be installed for use; the vertical directions of the top and bottom walls of the cells were directions arbitrarily selected parallel to one edge of the walls in question.

After the several plies had been built upon the form, the assembly was placed in an oven and subjected to heat to cure the vulcanizable elastomer plies. At the conclusion of the curing, the assembly was cooled and the plasterboard form broken up and removed through a hand hole left in one face of the cell.

Following are descriptions of the several plies, listed in the order in which they were plied up, i. e., the order in which they are found in the completed cell, going from the interior (fuel) side thereof to the outside.

I. Inner linen-A .005 inch thick nylon twill fabric (a) impregnated with a conventional adhesive dip, (b) having a spread coat of an elastomeric butadiene-acrylonitrile copolymer on each side and (c) having an elastomeric-butadiene-acrylonitrile skim coat on each side on top of the spread coat. Total thickness of the ply, .021 inch. The butadiene-acrylonitrile copolymer was compounded with conventional vulcanizing agents. In the cell construction, the fabric was disposed with its warp at 45 from the vertical, running from left at the bottom to right at the top of the respective faces to which it was applied.

II. Diusion barman-Applied by brush-coating of a solution of the following ingredients:

Parts Isobutene-acrylonitrile copolymer (prepared as above described) 75 Methyl phthalyl ethyl glycollate 25 Methyl ethyl ketone 900 This diffusion barrier solution was brush-coated over the entire exterior surface of ply I after it had been built upon the form. Three coats were applied in all, with drying at 60 C. for 3 hours after each coat before application of the succeeding coat.

Ill. A weak-wefted nylon cord fabric impregnated with a suitable adhesive dp.-On the side nearer the fuel, the fabric carried one spread coat and one skim coat of a butadiene-acrylonitrile copolymer composition. On the side further from the fuel, lthe fabric carried a Aspread coat and a skim coat of a butadiene-styrene elastomer stock. The elastomer stocks were compounded with conventional vulcanizing agents, and the cord was disposed in the directions assumed to be vertical for the respective cell faces. Total thickness of ply, .044 inch.

1V. Sea/mz: ply.-A single calendered sheet of natural rubber base, .110 inch thick, containing no reinforcing fabric. Contains reduced amounts of vulcanizing agents, so as to be only lightly cured.

V. Reinforcing ply.--A weak-wefted nylon cord fabric impregnated with a conventional adhesive dip, and having a spread coat and a skim coat of a butadiene-styrene elastomer compound on each side thereof. Vulcanizing agents were included for full vulcanization. The cords were disposed in the directions assumed to be horizontal for the respective faces.

VI. Renforcing and outside pIy.-A .006 inch thick nylon twill fabric impregnated with a conventional adhesive dip and having a spread-coat and a skim coat of a butadiene-styrene elastomeric copolymer on the side nearer the fuel and a spread coat and a skim coat of a butadiene-acrylonitrile elastomeric copolymer on the side farthest from the fuel. The warp in this ply was disposed at right angles to the warp in ply I.

TEST-ING The cell constructed as above described was tared and then filled three-quarters full of an aromatic aviation fuel. The cell was then mounted in a rocking machine which rocked the cell through an angle of 30 (15 on each side of the vertical) at a rate of 18 cycles per minute. The fuel was kept at 10G-120 F., and the rocking was continued for 25 hours. The fuel was removed, the interior of the cell blown out superticially dry, and the cell was reweighed. No measurable increase in weight was observed.

GUNFIRE TEST The cell was mounted in a supporting enclosure of a construction corresponding to the supporting enclosures provided for fuel cells in aircraft, said enclosure comprising walls of one-eighth inch aircraft aluminum sheet having riveted on the interior thereof three-quarter inch hat-sections of .064 inch thick aircraft aluminum sheet on four-inch centers so as to space the fuel cell from the walls. A resin-impregnated three-ply nylon fabric laminate with a weight of 0.41 pound per square foot was interposed between the`hat-sections and the fuel cell. The cell was filled three-quarters full of aromatic aviation fuel for the test.

The cell was then shot tested by firing four rounds of caliber armor-piercing ammunition therethrough at a range of 25 yards, the temperature being 20 F. and a tumble-board being interposed in the line of re to provide full tumble of the projectile at its point of entry into the cell. Two rounds were fired into one face of the cell in a direction normal thereto, and thereafter another two rounds were tired into an adjacent face in a direction normal to that face. Tabulated herewith are the par'- ticulars of the test.

Fuel head Condition of Wound Atter- Round at wound (mams) 1 minute 2 minutes 5 slight moisture.--. dry. 3% leak dry.

At the conclusion of the shot test, the fuel was removed, and the walls of the cell cut and peeled back in the areas surrounding the wounds to expose the diffusion barrier. ln no case was the barrier shattered beyond two inches from the wound. Moreover, the shattering was in a desirable pattern of concentric rings of cracks surrounding the wound, rather than radially extending cracks which might tend to propagate further away from the wound.

Fuel cells built as described above and installed in military aircraft gave satisfactory service over extended periods of time.

From the foregoing general discussion and detailed specic examples, it will be evident that this invention provides novel diffusion barriers for fuel cells, which barriers have excellent resistance to the diffusion of a wide variety of liquid fuels. The barriers have excellent gunre characteristics, in that they shatter over only relatively small areas in the vicinity of gunfire wounds. The barrier materials are soluble in various commercial solvents for convenient application during fabrication of the fuel cells, and have excellent adhesion to adjacent plies of such cells.

What is claimed:

l. A non-metallic fuel cell comprising an innermost s 2. A non-metallic fuel cell comprising an innermost lamina of a butadiene-acrylonitrile elastomeric copolymer composition in contact with the fuel therein, a sealant lamina of an elastomeriocomposition swellable by the fuel and disposed exteriorly of said innermost lamina, and a diffusion barrier membrane interposed between said innermost lamina and said sealant lamina comprising (A) a copolymer of isobutene and acrylonitrile containing from 10% to 40% by weight of isobutene, the balance being acrylonitrile, together with (B) from 15 to 50% methyl phthalyl ethyl glycolate, based on the total weight of said copolymer and said methyl phthalyl ethyl glycolate.

References Cited in the tile of this patent UNITED STATES PATENTS 2,430,931 Hershberger Nov. 18, 1947 2.437,966 Mowry Mar. 16, 1948 2,439,366 McLaughlin Apr. 6, 1948 2,497,123 Frolch Feb. 14, 1950 2,531,196 Brubaker et al Nov. 21, 1950 2,537,146 Lytton Jan. 9, 1951 2,537,626 Eberly et al. Ian. 9, 1951 FOREIGN PATENTS 613,932 Great Britain Dec. 7, 1948 OTHER REFERENCES Behavior of Plasticizers in Vinyl Chloride-Acetate 31'? Resins, by M. C. Reed, published in Ind. and Eng. Chemistry, vol. 35, p. 899, August 1943, TP978, R34. 

1. A NON-METALLIC FUEL CELL COMPRISING AN INNERMOST LAMINA OF A BUTADINE-ACRYLONITRILE ELASTOMERIC COPOLYMER COMPOSITION IN CONTACT WITH THE FUEL THEREIN, A SEALAND LAMINA OF AN ELASTIMERIC COMPOSITION SWELLABLE BY THE FUEL AND DISPOSED EXTERIORLY OF SAID INNERMOST LAMINA, AND A DIFFUSION BARRIER MEMBRANE INTERPOSED BETWEEN SAID INNERMOST LAMINA AND SAID SEALANT LAMINA COMPRISING A COPOLYMER OF ISOBUTENE AND ACRYLONITRILE CONTAINING FROM 10% TO 40% BY WEIGHT OF ISOBUTENE, THE BALANCE BEING ACRYLONITRILE. 